1. Field of the Invention
The present invention relates to multi-head printers and, more particularly, to techniques for processing print data for printing by multi-head printers.
2. Related Art
Various kinds of printers are well-known in the computer and digital imaging arts. Such printers include, for example, dot-matrix printers, laser printers, inkjet printers, and thermal printers. Thermal printers use thermal energy (heat) to produce printed output. More specifically, thermal printers typically contain a linear array of heating elements (also referred to herein as “print head elements”) that print on an output medium by, for example, transferring pigment from a donor sheet to the output medium or by initiating a color-forming reaction in the output medium. The output medium is typically a porous receiver receptive to the transferred pigment, or a paper coated with the color-forming chemistry. Each of the print head elements, when activated, forms color on the medium passing underneath the print head element, creating a spot having a particular density. Regions with larger or denser spots are perceived as darker than regions with smaller or less dense spots. Digital images are rendered as two-dimensional arrays of very small and closely-spaced spots.
A thermal print head element is activated by providing it with energy. Providing energy to the print head element increases the temperature of the print head element, causing either the transfer of pigment to the output medium or the formation of color in the receiver. The density of the output produced by the print head element in this manner is a function of the amount of energy provided to the print head element. The amount of energy providing to the print head element may be varied by, for example, varying the amount of power to the print head element within a particular time interval or by providing power to the print head element for a longer time interval.
A single thermal printer may include multiple thermal print heads, in which case the data to be printed is divided into a plurality of portions, referred to as “stripes,” each of which is printed by one of the print heads. The process of dividing the print data into stripes is referred to as “striping.” Multi-head thermal printers can be superior to single-head printers for cost and reliability reasons, particularly when wide printing is required. For example, the cost of a single wide head typically is significantly greater than the total cost of multiple small heads having the same aggregate width as the single wide head. Furthermore, the manufacturing yield for wide heads is very low compared to that of small heads. In addition, when a pixel fails in one print head in a multi-head printer, only the failing print head need be replaced, while the failure of a single printer in a single large print head requires the entire print head to be replaced at a much higher cost.
The print heads in a multi-head printer may be staggered with respect to each other. One example of this kind of printer is described in U.S. Pat. No. 4,660,052 to Kaiya et al., and is described as a heat-sensitive recording apparatus with multiple thermal heads disposed in a staggered arrangement along two platen rollers. The apparatus has alternate image segments printed on a first platen roller by a first set of print heads. The intervening segments are filled in by a second set of print heads printing on a second platen roller. The heads are arranged such that the printing of the second set of print heads overlaps the printing of the first set of print heads, forming “stitching” regions between each pair of adjacent segments in which the printing may be adjusted to obscure the presence of a transition from one to the other.
The use of stitching regions may create undesirable visible artifacts in the printed image if adequate preventative steps are not taken. Various techniques have been employed to “stitch” image segments within stitching regions so that the presence of the stitching regions is imperceptible to the greatest extent possible. Stitching techniques include techniques for performing image processing on stitched image segments prior to printing, mechanical techniques for properly printing stitched image segments with proper alignment, and combinations thereof. Particular examples of stitching techniques may be found, for example, in a commonly-owned patent application Ser. No. 10/374,847, filed on Feb. 25, 2003, and entitled “Image Stitching for a Multi-Head Printer.”
Striping and stitching are merely two examples of kinds of processing that may need to be performed on print data before it is provided to the print heads for printing. As the speed of multi-head printers continues to increase and as price competition among printer manufacturers continues to increase, it is becoming increasingly important that techniques for performing striping, stitching, and other image processing techniques be capable of processing print data both quickly and inexpensively.